U.S. patent number 4,549,998 [Application Number 06/701,446] was granted by the patent office on 1985-10-29 for contacting device.
This patent grant is currently assigned to Imperial Chemical Industries PLC. Invention is credited to John E. Porter, Colin Ramshaw.
United States Patent |
4,549,998 |
Porter , et al. |
October 29, 1985 |
Contacting device
Abstract
In a centrifugal device for contacting a liquid with a gas or
with a second liquid, high rates of mass transfer are achieved by
carrying out the contacting on a rotating plate whose surface is
capable of creating perturbations in liquid flowing across it.
Inventors: |
Porter; John E. (Newcastle upon
Tyne, GB2), Ramshaw; Colin (Norley, GB2) |
Assignee: |
Imperial Chemical Industries
PLC (Hertfordshire, GB2)
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Family
ID: |
10526116 |
Appl.
No.: |
06/701,446 |
Filed: |
February 13, 1985 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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444336 |
Nov 24, 1982 |
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Foreign Application Priority Data
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Nov 24, 1981 [GB] |
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8135407 |
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Current U.S.
Class: |
261/88 |
Current CPC
Class: |
B01D
11/0453 (20130101); B01J 19/0066 (20130101); B01D
11/0461 (20130101); B01J 19/1806 (20130101); B01J
19/0073 (20130101) |
Current International
Class: |
B01J
19/18 (20060101); B01D 11/04 (20060101); B01J
10/00 (20060101); B01F 003/04 () |
Field of
Search: |
;261/88,89,90 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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977673 |
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Nov 1975 |
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CA |
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808114 |
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Jul 1951 |
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DE |
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2308440 |
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Aug 1974 |
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DE |
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2408753 |
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Apr 1975 |
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DE |
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347303 |
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Apr 1931 |
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GB |
|
757149 |
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Sep 1956 |
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GB |
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Primary Examiner: Miles; Tim
Attorney, Agent or Firm: Cushman, Darby & Cushman
Parent Case Text
This is a continuation of application Ser. No. 444,336, filed Nov.
24, 1982, now abandoned.
Claims
We claim:
1. In a centrifugal device for contacting a first liquid with a gas
or a second liquid comprising: a rotor having a plurality of plates
mounted transversely to and coaxially with the axis of rotation of
said rotor and being spaced apart along said axis, at least one
surface of each of said plates being capable of creating
perturbations in any liquid flowing across it; means to charge said
first liquid to a radially inner part of said rotor; means to
charge said gas or second liquid to said rotor; and means to
collect liquid discharged from said rotor, the improvement wherein
the said plates are spaced apart by a distance of 0.2 to 5 mm along
said axis and wherein at least a portion of said perturbations are
caused by a plurality of continuous substantially V-shaped channels
in said surfaces, the radially outer surface of each of said
channels being steeper than the radially inner surface thereof.
2. The centrifugal device of claim 1, having drive means able to
rotate said plate at a speed sufficient to subject any liquid
flowing across it to a mean acceleration of more than 10
meters/second.sup.2.
3. The centrifugal device of claim 2, wherein said plate capable of
creating perturbations is porous.
4. The centrifugal device of claim 2, wherein said plate capable of
creating perturbations is perforated.
5. The centrifugal device of claim 1, wherein:
said radially outer surface of said channels is of continuously
decreasing gradient away from said axis of rotation of said
rotor.
6. The centrifugal device of claim 5, wherein the pitch of said
plurality of channels is between 1 mm and 20 mms.
Description
THIS INVENTION is concerned with centrifugal liquid-liquid and
gas-liquid contacting devices.
Various centrifugal devices for contacting a first liquid with a
second liquid or gas have been proposed, including that described
in UK Patent Specification No. 757,149. The device described in UK
Patent Specification No. 757,149 comprises a rotor in which are
disposed filler bodies or plates, over the surface of which filler
bodies or plates the first liquid and/or the second liquid or gas
flows under centrifugal force.
We have now found, where one or more plates is or are mounted
substantially transversely to the aforesaid axis, that the use of
one or more plates which is or are capable of creating
perturbations in the film of liquid flowing thereover gives an
improvement in mass transfer between the first liquid and the
second liquid or gas.
Accordingly, the present invention provides a centrifugal device
for contacting a first liquid with a gas or a second liquid, which
device comprises a rotor having one or more plates mounted
transversely to the axis of rotation of the rotor, means to charge
the first liquid to a radially inner part of the rotor, means to
charge the gas or second liquid to the rotor and means to collect
liquid discharged from the rotor, characterised in that a surface
of at least one plate is capable of creating perturbations in any
liquid flowing across it.
The flow of the gas or second liquid through the rotor in a device
of the present invention may be co-current with or counter current
to the flow of the first liquid although counter current flow is
often preferred. Where counter-current flow is employed in the
present invention it will be appreciated that the density of the
gas or second liquid is less than the density of the first
liquid.
As examples of perturbations which may be created in the first
liquid in a device of the present invention we would mention (a)
formation of a spray of fine droplets of the first liquid which is
ejected from the surface of a plate and (b) continuous or periodic
renewal of the surface of the thin film as it flows across the
surface of a plate while remaining in contact therewith over
substantially all of the surface of the plate. The aforesaid
contact may be maintained, for example, by adapting the plate such
that under appropriate conditions the "Coanda" effect may be
utilised to retain the first liquid on the plate until it reaches
the outer perimeter of the plate.
The plate surface capable of creating perturbations in any liquid
flowing across it may take a variety of forms in order to give that
effect. Thus the plate may have protrusions from the surface or
indentations in the surface or the plate may be corrugated, porous
or perforated. These features designed to create perturbations are
preferably disposed substantially transversely to the flow of the
first liquid across the plate surface. More preferably, they are
disposed in one or more circles which are concentric with the axis
of rotation of the rotor.
Thus in one form, where the surface features are one or more
channels in the plate surface, it is preferred that they are
continuous channels disposed concentrically about the axis of
rotation of the rotor. Such channels are preferably substantially
V-shaped and in particular the radially outer surface of the
V-shaped channel is preferably steeper than the radially inner
surface thereof. More preferably the upper portion, at least, of
the radially outer surface presents a continuously decreasing
gradient to the first liquid as it emerges from the channel; such a
decreasing gradient affords retention of the first liquid on the
surface of the plate at higher rotational speeds.
In another form of the device according to the present invention,
the plate of which the surface is capable of creating perturbations
in any liquid flowing across it is porous--or at least a portion of
that plate is porous. Use of such a porous plate has the advantage
that a first liquid which is fed to the first surface of the plate,
permeates through the plate to appear on the second surface thereof
and is thus exposed to the second liquid or gas on both surfaces of
the plate. Thus the porous plate may be inter alia foraminate,
cribriform or gauze-like. It is particularly preferred that the
plate, or at least a part of it, be perforated. The perforations
are preferably disposed symmetrically about the axis of rotation of
the rotor, for example in one or more circles disposed
concentrically about the axis of rotation. The perforations may be
of uniform size throughout the perforated area of the disc or may
vary; for example, the size of the perforations may vary with
distance from the axis of rotation.
Whilst it is often preferred that the thin porous plate is rigid,
for example it is made of metal or plastic, we do not exclude the
possibility that it may not be self-supporting and may become
plate-like only when rotated at a sufficiently high speed. For
example, it may be made from a woven, knitted or so-called nonwoven
fabric.
When the surface features creating perturbations, for example
corrugations, protrusions, indentations or perforations, are
disposed in concentric circles, the circles are preferably spaced
at a density, measured in a radial direction, of between 50 and
1,000 per meter, preferably more than about 100 per meter. Thus the
pitch of the pattern of these surface features, that is the
distance between repeated features of the pattern, is preferably
between 1 mm and 20 mms, more preferably less than about 10 mms.
When the surface features creating perturbations are channels, the
depth of each channel is preferably between 0.05 and 5 mms,
especially between 0.2 and 5 mms and more especially between 0.5
and 2.5 mms. Very shallow channels, for example of the order of
0.05 to 0.25 mm, may if desired be formed by etching the plate
surface.
The thickness of the plate or plates employed in the device
according to the present invention is generally between 0.05 and 5
mms, depending upon the material of construction, the specific
contacting duty to be carried out and the form of surface features
chosen. While the thickness of the plate may vary--and obviously
will vary with some forms of surface features--in general when
referring to plate thickness we refer to the plate thickness as it
would be without those features. The plate thickness is preferably
between 0.25 and 1.5 mms, especially between 0.5 and 1.0 mm.
The outer diameter of the one or more plates used in a device of
the present invention is typically in the range 25 cm to 5 meters
and is preferably about 1 meter and where the one or more plates is
in the form of an annulus the inner diameter thereof is typically
in the range 5 cm to 1 meter.
The material of construction of the one or more plates used in a
device of the invention should be such that the or each plate can
withstand the stress generated in the material during use.
Preferably the material is substantially resistant to attack by or
reaction with the material with which it may be in physical
contact. Typically, the material from which the plates are formed
is a glass, ceramic or preferably a metal, more preferably a
chemically resistant metal, e.g. stainless steel, nickel or
titanium. It is often desirable to give the plates an appropriate
surface treatment, which may be chemical, e.g. etching, or
physical, e.g. sand-blasting, to provide surfaces which are wetted
by the liquid.
Where a device of the present invention comprises a plurality of
plates, they are disposed along the axis closely adjacent to one
another to form narrow passages and preferably the mean axial depth
of the passageways between adjacent plates is less than about 50
mms and more preferably is between 0.2 and 5 mms. Where the axial
depth of a passageway varies along the radial length thereof, for
example both of the two opposing surfaces which define the
passageway have peaks and troughs, the troughs of the first of the
said surfaces being aligned with the troughs of the second of the
said surfaces and the peaks of the first surface being aligned with
the peaks of the second surface, the narrowest gap is often about 2
mm and the largest gap is often about 8 mm.
Where the device of the present invention comprises a plurality of
plates it is often preferred that they are so arranged and/or
shaped that the axial depth of each passageway between adjacent
plates is not constant. In particular, it may be preferred that the
surface contours of one plate "engage" the contours of the opposed
surface of an adjacent plate in the sense that protrusions on one
surface are aligned with protrusions on the surface opposed
thereto. By this means a fresh spray of the first liquid can be
continuously formed since as the first liquid flows through the
passageway between two plates, the opposed surfaces of which have a
multiplicity of contours, it is ejected from a protruding contour
of one plate, deposited on a contour on the opposed surface of the
adjacent plate, from a protruding contour of which it is rapidly
ejected and this ejection alternates between the protruding
contours of the two opposed surfaces as the first liquid flows
along the passageway therebetween.
Means to charge the first liquid to the one or more plates is
conveniently a stationary liquid feed pipe provided with suitable
orifices and arranged co-axially with the rotating one or more
plates.
Where counter-current flow is employed in the present invention,
means to charge the gas or second liquid to the one or more plates
is provided adjacent the outer perimeter of the plates.
Conveniently the one or more plates are mounted in a stationary
housing into which the gas or second liquid is fed at a pressure
sufficiently high to overcome the pressure of the liquid discharged
from the perimeter of the one or more plates.
The first liquid or a portion or derivative thereof is conveniently
collected in a stationary housing in which the one or more plates
are mounted and from which it can be withdrawn when desired.
The mean acceleration to which the first liquid is subjected in a
centrifugal device of the present invention is more than 10
meters/second.sup.2, preferably more than 100 meters/second.sup.2
and more preferably more than 1,000 meters/second.sup.2. Generally,
as the mean acceleration is increased the rate of mass transfer
between the first liquid and the gas or second liquid is increased.
However, it will be appreciated that for plates of a certain
diameter, loaded at a certain liquid flow rate, the power
requirements of the apparatus in which the plates are mounted is
proportional to the square of the speed of rotation of the plates.
Thus the optimum speed at which the plates are rotated often
represents a commercial balance between the desirability of a high
mean acceleration and a low power requirement.
Mean acceleration a.sub.m is defined by the equation ##EQU1## where
N is the rotational speed of the one or more plates about the axis
of rotation thereof in revolutions per minute, r.sub.o is the
distance in meters from the aforementioned axis to the radially
inner edge or edges of the one or more plates and r.sub.1 is the
distance in meters from the aforementioned axis to the radially
outer edge of the one or more plates. It will be appreciated that
where a plate is in the form of a disc, r.sub.o, in the calculation
of the mean acceleration to which a liquid flowing thereover is
subjected, is zero.
The rate of flow of liquid across the surface of a plate is
typically in the range 10.sup.-5 to 10.sup.-2 meters.sup.3
/second/meter of plate perimeter.
Devices according to the present invention may be employed in inter
alia absorption, desorption, counter-current extraction,
distillation and homogenisation processes.
The present invention will now be further illustrated by reference
to the accompanying drawings which show, by way of example only,
one embodiment of the present invention. In the drawings:
FIG. 1 is a vertical sectional view of a gas-liquid contacting
device according to the present invention;
FIG. 2 is a horizontal sectional view on the line AA of FIG. 1;
FIG. 3 is a detail, on an enlarged scale, of two opposing surfaces
of the plates of FIGS. 1 and 2; and
FIGS. 4 to 7 illustrate improved oxygen transfer rates which can be
obtained with a centrifugal device of the present invention.
In FIGS. 1 to 3 of the drawings, a rotor, designated generally by
the numeral 1, is mounted upon a drive shaft 2, by means of which
it is rotated within a chamber 3 defined by cover 4, base 5 and a
cylindrical sidewall 6. Where the shaft 2 passes through the base 5
a conventional mechanical seal 7 is provided. Rotor 1 comprises a
plurality of thin annular plates 8, mounted on six pins 9 on the
base 10 of the rotor and spaced apart by washers 11, and a cover
plate 12 which is provided with a liquid seal 13. The chamber 3 is
provided with a gas feed-tube 14, a gas discharge tube 15, liquid
feed-pipe 16 which has apertures 17 in its lower end and liquid
discharge port 18.
In operation of the illustrated device, the rotor 1 is rotated; a
liquid is fed via feed-pipe 16 and the apertures 17 to the stack of
annular plates 8 and it moves radially outwards across the surfaces
thereof as a thin film. The thin film flows across the horizontal
surfaces 19, down the sloping surfaces 20, at the bottom of which
it is thoroughly mixed, up the vertical surfaces 21 and, because of
the chamfered edge where surfaces 19 and 21 join, is retained on
the surface by the Coanda effect to continue its radially outward
flow. The liquid is ejected from the outer perimeter of the plates
into chamber 3 from where it runs off through discharge port 18. A
gas is fed into the device through the gas-feed tube 14, it enters
the passageways between the annular plates 8 and moves radially
inwards to leave the passageways at the inner edge of the plates.
The portion of the gas which is not absorbed by the liquid is
discharged through tube 15.
Various aspects of the present invention will now be described by
reference to the following Examples, which are illustrative of the
invention.
EXAMPLE 1
Water, at flow rates of 300 to 950 mls/second, was charged adjacent
the axis of rotation to a plain disc of 600 mms diameter, rotating
at 100 rpm in an atmosphere of nitrogen. The concentration of
oxygen in the water feed was 8.15 ppm and the concentration in the
water at the disc perimeter was measured; from the measured
concentrations, the rates of oxygen transfer from the water to the
nitrogen from the axis of the disc to its perimeter were
calculated. The results are expressed graphically as Curve A in
FIG. 4.
The exercise was repeated using a grooved disc, of 600 mms
diameter, having a profile corresponding to one of the discs shown
in FIG. 3. The groove depth was 2 mms and the pitch 5 mms. The
results are shown as Curve B in FIG. 4.
The exercise was again repeated, this time using a perforated disc
of 600 mms diameter. The perforations were in the form of 1.5 mm
holes punched in a triangular pitch such that the hole area
amounted to about 30 percent of the disc surface area. The results
are shown as Curve C in FIG. 4.
As is clearly seen from FIG. 4, the rate of transfer of oxygen from
the water is greater on the grooved disc than on the plain disc and
is much greater still using the perforated disc.
EXAMPLE 2
Example 1 was repeated using the same three discs and the same
conditions throughout, except that the discs were rotated at 200
rpm. As shown in FIG. 5, enhanced results were obtained with all
three discs and again the two discs having surfaces designed to
create perturbations in the liquid flow gave markedly better
results than the plain disc. (Curve lettering is as before).
EXAMPLE 3
The procedure of the previous two Examples was repeated, using this
time only the grooved and the perforated discs and rotating the
discs at 400 rpm. FIG. 6, in which Curve B is for the grooved disc
and Curve C for the perforated disc, again shows the high oxygen
transfer rates obtainable.
EXAMPLE 4
A further oxygen transfer Example was carried out, using the
procedure of the previous Examples but this time using "twinned"
discs, rotated at 700 rpm.
In a first experiment, the discs were two 600 mm diameter discs;
the opposed surfaces of the discs had profiles corresponding to
those shown in FIG. 3 in which the depth of the grooves was 2 mms
and the pitch of the grooves was 5 mms. The discs were mounted with
the parallel portions of the facing surfaces spaced 2 mms apart.
Nitrogen was charged to the space between the discs, cocurrently
with the water, at a rate of 375 mls/second. The calculated oxygen
transfer rate is shown as Curve B in FIG. 7.
Results obtained with a pair of plain 600 mm discs, also spaced 2
mms apart, are included for comparison as Curve A of FIG. 7.
* * * * *